Radio frequency identification systems (RFID) are a form of wireless communication that utilizes radio waves to identify and track objects. Each tag carries a unique identification number; which is programmed at the time of manufacturing to ensure the object carries a distinctive identity and description. Conventional RFID systems, such as the one shown in FIG. 1, typically include a reader (or an interrogator) and a tag (or transponder). The tag includes a microchip that stores data and an attached antenna. FIG. 2 illustrates a prior art RFID tag with a simple dipole antenna structure 3. The tag 1 includes a chip 2 coupled to the dipole antenna structure 3, which includes of a pair of antenna elements 4 and 5 supported on a substrate 6.
RFID tags can be attached to an object, i.e. substrate, and can store or transmit information concerning the object, such as a unique identifying number, object status such as opened or unopened, location, and the like. There are two different methods of communicating with an RFID tag—near-field and far-field, with the main difference between the two methods being the reading distance. Near-field communication is conventionally defined as having distances of less than 1.5 m, while far-field communication is more than 1.5 m. Additionally, RFID tags may be passive, active, or semi-active.
Near-field communication (NFC) transmits data either through inductive coupling between the reader and the tag, or through capacitive coupling, with inductive coupling being more popular in use. Inductive coupling involves the use of a magnetic field to energize the RFID tag. A magnetic field is created in the near-field region that allows the RFID reader's antenna to energize the RFID tag, which then responds by creating a disturbance in the magnetic field that the reader will then detect. Capacitive coupling is less common than inductive coupling in NFC and utilizes a quasi-static electric field between the reader antenna and the tag antenna.
Far-field communication (FFC) involves sending and receiving electromagnetic (EM) waves, typically through the use of capacitive coupling (or propagation coupling). The reader transmits a signal that is then reflected off the tag and returned to the reader. By modulating the load on the tag, data can be encoded in the modulating reflected signal. Compared to NFC, the reading distance for FFC is typically more than 1.5 m.
Passive tags have no power source and instead draw power from the field created by the reader and use the energy from the field to power the microchip's circuits. With passive RFID, the RFID tag is irradiated with radio frequency waves from the RFID reader. The RFID tag uses the energy from the radio frequency waves to emit an RFID signal, containing the RFID tag identification location or other data, back to the RFID reader. The RFID reader then receives the RFID tag information and software can be used to interpret the information on the tag, such as calculate a tag's location.
Active tags have a power source and broadcast their signal at set intervals rather than relying upon signals from the reader. Active RFID tags have an independent onboard power source, such as a battery, or are connectable to one. Active RFID tags can transmit the Radio Frequency signal autonomously, at a selected time or at programmed triggers, for example, from a temperature sensor. The power source on active RFID tags also gives them a longer range than passive RFID tags.
Semi-active tags, like active tags, also have a power source, but differ in that they wait for a reader to communication with them, similar to passive tags. Also similar to passive tags, these tags utilize the power from the reader's transmission to communicate back to the reader. Compared to passive tags, semi-active tags contain more complex electronics and are thus more expensive, but can be read from a farther distance, faster and through opaque materials.
While there are different types of RFID systems, in most systems the reader sends out electromagnetic waves, which the tag is designed to receive. Depending on the structure, the RFID reader can identify items that are anywhere from a few centimeters to several meters away. The size of the RFID tag's internal antenna is one indicator of the tag's range. Generally, small RFID tags contain small antennas and shorter read ranges, while large RFID tags that contain larger antennas have longer read ranges. Additionally, RFID antennas can be strongly influenced by their surrounding environment. Water can absorb and reflect RF energy and therefore may decrease an RFID system's performance, including read ranges and read rates.
RFID technology has been used in various capacities. For example, RFID technology may be used for identification of products. The microchip in the RFID tag may contain information that aids in identification of the item to which the RFID tag is attached. Unlike bar codes, which require direct line-of-sight for access (i.e., the bar code needs to be visible in order to be scanned), RFID tags can be read by the RFID reader without the need for direct line-of-sight. RFID tags also have greater capabilities than bar codes because much more information can be stored on the RFID tag than can be printed on a bar code.